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Properties that affect the Dust Explosion
Hazard
Critical
parameters for dust explosions
| Particle size |
< 0.1 mm |
| Dust concentration |
40 g/sqm - 4000 g/sqm |
| Moisture content |
< 11% |
| Oxygen |
> 12% |
| Ignition energy |
> 10 mJ - 100 mJ |
| Ignition temperature |
410° - 600° C |
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There are a few main factors that influence the combustibility of
a dust, many of them are still not understood very well, and usually
the only way to get data concerning a specific dust is to do analytical
tests on the substance. Most books on dust explosions have data
for the minimum explosive concentrations and other properties of
common powders. Discussed briefly here are the different factors
that affect the explosibility of a dust, this is not meant as a
complete guide but should point to main trends in behaviour.
Dust Chemistry and Moisture Content
If it isn't combustible there can't be an explosion. Dust chemistry
is affected by the elements that make up the molecule and their
specific configuration in the molecule. Dust chemistry is one
of the really fundamental considerations when investigating dust
explosions. It directly influences the thermodynamics, how much
heat is released, and the kinetics, how fast heat is liberated,
of the reaction, which in turn directly affect the severity of
the explosion. The heat of combustion per mole of O2 used is a
commonly available and useful number for calculating the total
heat release in a dust explosion.
Data is usually available in tables specifying these quantities,
however short of looking it up or actually performing experiments
in a Hartmann Bomb, it is very difficult to predict quantitatively
(it is beyond the scope of this website, although there are further
details in R. Eckoff's "Dust Explosions in the Process Industries")
just how explosive a given dust will be.
Moisture content of a dust will affect the ability of a dust
cloud to be ignited and its ability to sustain an explosion. Increasing
moisture content pushes the ignition energy up exponentially with
some dusts. The moisture works in several main ways. The heating
and evaporating the moisture provides an inert heat sink. Once
evaporated the water vapour mixes with pyrolysis gases and makes
them less reactive, and can also increase intermolecular cohesion
of the dust meaning a larger effective particle size.
Particle Size and Specific Surface area
No matter how combustible the powder, if it's in big lumps it
isn't going to cause a dust explosion. Although there is a clear
dependence on size and surface area of dust particles, it does
not vary linearly with the explosibility of the powders. Often
the dust (i.e. coal dust) will become more and more explosive
down to a certain size limit at which it will plateau. The reason
that surface area affects the violence of explosion is that particle
size/surface area influences the speed at which volatiles are
extracted from the particle (or how fast the particle vaporises)
before they burn. As long as this is the limiting factor in the
combustion, reducing the particle size will increase the severity
of explosions. As soon as the speed of gas phase mixing or actual
combustion of volatiles is the limiting factor particle size becomes
irrelevant (although usually this is at sizes much less than 50m).
This limit can also be different for high dust concentrations
where the explosions are more violent.
Dust Concentration
Dust cloud explosions can only occur if the dust concentration
is within certain limits. This is analogous with the concept of
upper and lower flammable limits of mixtures of gas (or vapours)
and air. In general the lowest concentration of dust that can
give a dust explosion is around 50-100g/m3 and the maximum is
2-3kg/m3. These limits are dependent on the particular chemical
in question and on the particle size distribution. The worst cases
are usually when the dust concentration is slightly above the
stoichiometric concentration. Upper concentration limits are dictated
by the minimum amount of oxygen needed for explosion, lower limits
by the minimum quantity of particles needed to sustain a combustion.
Turbulence
A more turbulent cloud will result in a more severe explosion
as, due to the more homogenous concentrations and lower degree
of dispersion, the flame front will move more quickly through
the dust cloud. However a less turbulent cloud is more easily
ignited, as heat dissipation is at a lower rate so the initial
heat release is more locally concentrated, leading to a higher
probability of ignition from an input of energy.
Oxygen Content of Oxidising Gas
Less oxygen in the air causes the explosion to be much less severe
as it limits the rate of combustion of the dust, thus limiting
the oxygen in process vessels can minimise the possibility of
a dust explosion (fire can only be sustained if oxygen concentration
is greater than 10% in air).
Degree of Dust Dispersion
Dispersion and degree of agglomeration affect the combustion as
they change the effective local dust concentrations and the effective
particle size respectively. A more evenly dispersed dust will
burn more easily. The degree of dispersion is usually dependant
upon the way method of dust dispersion and the turbulence in the
system.
Initial Dust Cloud Temperature
At higher initial temperatures the dust cloud is more easily ignited
and the minimum dust concentration required for explosion is lowered.
However the maximum explosion pressure is lowered as the oxygen
is of a lower concentration per unit volume when the temperature
is higher, so less combustion can occur.
Initial Pressure of Dust Cloud
Increasing the pressure in a dust cloud makes the explosion more
violent, because essentially all of the combustibles are closer
together. It also lowers the required ignition energy.
Combustible Gas Mixed with Dust Cloud
Addition of a fuel gas (or vapour) can lower the ignition energy
for a pure dust cloud massively, and raise the maximum explosion
pressure.
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